There are different methods for performing a transfer printing on tri-dimensional articles:
In-Mould Decoration (IMD)
Two different methods are used for performing such printing depending on the structure of the surface to be printed, noteably, Film Insert Moulding (FIM) and In-Mould Labelling (IML).
FIM is generally used to print and mould items with a slight hollow such as mobile phone shell, touch screen panels, etc.
IML is mainly used to mould printed labels on plastic containers used for food packaging, such as ice cream boxes, cheese, etc.
The injection and forming devices needed for the FIM and moulding tools and equipment for the IML are very expensive and make these techniques adequate only for high volume production runs. Another drawback with these printing systems is that each item requires a specific mould.
Cubic Printing (Dip Coating)
Cubic printing method is a method that allows a print to be transferred onto a tridimensional plastic material, especially large hollow articles that can be easily crushed, by way of a special film that leaves a floating layer of inks into which the article to be decorated is immerged.
Cubic printing method needs specialized tooling and equipment, such as a water basin where the film is floating and the article is placed on the film. This process is acceptable only for graphic elements that do not need a high accuracy in the positioning of the print, typically a pattern that repeats itself. Given the high set-up costs, this process is only cost-effective for high volume production runs.
Digital 3D Sublimation Using an Ink-Jet Printable Film
A thermally formable film especially designed to transfer any graphic image onto a tri-dimensional article is printed using dye and pigment based sublimation inks with standard inkjet piezo technology. Unlike the sublimation papers, which are stiff and dimensionally stable under heat and pressure, coated thermally formable film is designed to take the shape of the article that it is adhered to and to sublimate the image onto the surface of that article.
Generally, the transfer is made on plastic materials (such as PET, PA, PBT, etc.) with melting temperatures above the maximum transfer temperature of 210° C.
It is also possible to transfer the printing onto any type of surface, i.e., wood, metal, glass etc. provided that they are previously coated with a layer of polyester varnish. During the transfer process the ink that is on the top layer sublimates, passing from solid to gaseous state, and thus penetrates the surface of the article to be imaged, making it scratch resistant.
This process requires two steps: a first step involving the printing of an image onto a transfer medium, and a second step involving the transfer of the printing onto the article.
In the first step, this type of printing can be done using a number of methods, screen printing, flexographic printing, offset printing or inkjet printing, as long as the printing is done using sublimation inks. The different type of printing method is due to the equipment already available at the printing shop and the number of copies to be printed that make the specific method cost effective.
In the second step, the important issues are that the medium is thermo formable and that it produces an image with the highest resolution without producing distortions and areas voids of ink. As it is known, this second step is performed by introducing the material into an oven and applying vacuum between the surface of the material and the article to be printed. In the oven, the material can heated in two different ways: the first manner is what is normally referred to as a “convection oven” where the heat is produce by resistive heating elements and the temperature is kept constant through a regulated air flow for even heating of the material. The second type of heating of an oven is an infrared rays (IR) heating system in which infrared lamps heat the material directly with a very even distribution of the heat.
The base product used for making this transfer medium is known on the market as APET (amorphous polyethylene terephthalate). The APET film to be heated in IR ovens is typically metalized to provide a sufficient barrier between the gaseous ink and the APET film in order to prevent the migration towards APET film of the ink gas developed during the sublimation process with resulting reduction of the quality of the transferred image and waste of ink. In addition, the metallic layer improves the adhesion of the ink retention coating.
The shortcomings of the metalized layer are that the operator cannot see through such layer and it becomes difficult to register accurately the position of the article and the printed image. The advantage of the metalized layer is that it allows a faster and more even heating of the APET in ovens utilizing IR heating system.
In the case in which convection ovens are used, another option is the utilization of a base medium made of APET 160 to 250 microns in thickness with various coatings as described in EP 1392 517 B1.
In both the cases, the transfer film has to be coated/treated for the following reasons. The first reason is so that the transfer film molds itself to conform to the shape of the article to be printed when heated and put under vacuum. The second reason is so that the transfer film retains the sublimation ink and releases it once the sublimation ink has been heated and has reached its gaseous state. The third reason is so that the transfer film releases the ink with the least amount of ink waste. The fourth reason is so that the transfer film releases the ink with the best image definition and color gamut fidelity. The fifth reason is so that the transfer film allows the evacuation of the vapor that is formed during the sublimation process between the film and the surface to be printed without forming any gas bubbles that will result in unprinted areas.
Of all these requirements, the most difficult to meet, are the last two, because they are technically in conflict since, in order to obtain the best possible image definition, it is necessary that the distance between the transfer film and article to be printed is minimal.
With a perfect contact, the ink vapors pass directly from the coating of the transfer film to the surface of the article to be printed maintaining the original definition without any “bleeding.” That is typically the process that has been used for sublimation printing of flat surfaces using special papers for sublimation printing already known.
However, in the case of tri-dimensional printing, it is necessary to use a thermo-formable plastic material as the substrate to manufacture the transfer film, and the moist air that is trapped between the two surfaces once it is heated and becomes a vapor, occupies a larger volume and, not being able to escape, it forms bubbles between the thermo-formable plastic material and the surface of the article.
These bubbles, depending on their size, reduce up to neutralizing the ink vapor migration, thus compromising the definition and the fidelity of the image up to a point where there is no transfer of ink at all.
The different patents that have been developed, such as European patent EP 1.102.682.B1 and U.S. Patent Publication No. 2009/0068383 A1, have pointed out the necessity of a coating with a surface roughness defined in BEKK (less than 50″) that allows a good image definition and permits the air to escape.
These different coatings described have formulations based principally on the dispersion of a pigment system in one of many resins used as a binder and as a receptor for sublimation ink. It is the choice of the type of pigment system and its granular dimension that determines the roughness of the surface. All of these publications describe the use of silica in different dimensions but calibrated to maintain the surface roughness desired. The problem derived from the use of these silica particles is that they are an inefficient manner for air dissipation. Additionally, since the silica provides the important function to absorb the ink in its liquid phase and than release it during the sublimation cycle, the formulations described only release a partial quantity of that ink during the sublimation cycle.
The effort to limit as much as possible the loss of ink during the transfer phase, requires that the APET thermo formable film is isolated by an appropriate treatment from the flow of ink vapors during the transfer. This barrier treatment is normally obtained through a deposition of thin layer of metal (i.e. aluminum), through a sputtering process or vacuum metallization. This technology allows to obtain good results in terms of barrier and to optimize the performance of ovens using IR technology but it has shortcomings in terms of cost, fragility of the thin metal layer as the base for further coatings and finally, but most importantly, the metal blocks the light. This last factor makes it difficult to track and position the film over the article to be printed.
Many of the resins that are of common knowledge and that are utilized as a primer for further coating have little compatibility with the aqueous based top coating and a high level of absorption for sublimation inks, so that they will not work as a barrier.